metallicity

Metallicity is a measure of the proportion of "heavy elements" or "metals" (in astronomy,
elements heavier than hydrogen or helium) that a star contains. Usually,
metallicity is given in term of the relative amount of iron and hydrogen
present, as determined by analyzing absorption
lines in a stellar spectrum, compared with the solar value. The ratio
of the amount of iron to the amount of hydrogen in the object is divided
by the ratio of the amount of iron to the amount of hydrogen in the Sun.
This value, denoted as [Fe/H], is calculated from the following logarithmic
formula.

For example, if the metallicity [Fe/H] = -1 then the abundance of heavy
elements in the star is one tenth that found in the Sun; if [Fe/H] = +1,
the heavy element abundance is 10 times the solar value. Measurements for
thousands of stars have established that the range of values for [Fe/H]
is from -4 (very metal-poor) to
+1 (very metal-rich).

Some general observations about the characteristics of stars as indicated
by their metallicities: 1) the disk portion
of a galaxy has a range of metallicities, with Population
I stars having values > -1, i.e., towards smaller negative numbers to
positive numbers less than +1, whereas Population
II stars have negative values beyond - 1; 2. globular
clusters and halo stars are
metal-poor (values more negative than -1); 3) metal-rich stars are in the
red segment of the color index and metal-poor
stars are blue; 4) although there can be complexities, in general metal-poor
stars are young in appearance (either near the outer limits of the Universe
which show stars that formed in the first few billion years after the Big
Bang or stars formed more recently from gas clouds that have had little
contribution of heavier elements from supernovae) and short-lived; 5) metal-rich
stars from F, G, K, and M positions on the main sequence are redder than
stars of similar sizes (masses); and 6) dust around a star will make it
redder.

Overall, the rule of thumb is just that a star will show a metallicity that
depends on prior processes that have changed the composition of the interstellar
gas in the neighborhood in which it forms. This is a function mainly of
the number of supernovae that have occurred
previous to the formation of the star and the amounts of metals each ejected
that then became mixed into the cloud that supplies the star (and other
stars growing from this cloud). Since, over time the gas composition in
the interstellar medium should progressively enrich in metals, then those
stars that are metal-rich tend to have organized in later stages of a galaxy's
history.

From the above it follows that stars that are extremely metal-poor are likely
to be first-generation and thus primitive. Moreover, only stars that have
a fairly high metallicity (roughly solar or greater) are possible candidates
for planetary systems, since the cores of planets are formed from metals
such as iron and nickel.